1. Field of the Invention
The present invention relates to an oscillator circuit, and more specifically to a voltage control oscillator for use in a phase locked loop (PLL) circuit to cover a wide range of oscillation frequency within a predetermined voltage range.
According to the present invention, an oscillator circuit comprises a resonator including an inductive resonator such as a coil, an active element such as a transistor and IC, and a variable capacitance diode. With the configuration, a direct current control voltage (hereinafter referred to as a control voltage) is externally provided for the variable capacitance diode through a bias circuit. The control voltage is changed to vary the capacitance value of the variable capacitance diode, and finally to vary the oscillation frequency of the oscillator circuit.
With the voltage control oscillator, the impedance element of the bias circuit is divided into two or more elements. A capacitance element is connected between earth and the connection point among the two or more elements. The values of the impedance elements and the capacitance elements are set such that the relation between the cut-off frequency f1 of a first low-pass filter comprising the impedance element near the variable capacitance diode and the capacitance element and the output oscillation frequency f0 of the voltage control oscillator is f0&gt;f1, and the relation between the cut-off frequency f2 of a second low-pass filter comprising the impedance element near the input terminal of the control voltage and the capacitance element and the cut-off frequency f1 of the first low-pass filter is f1&gt;f2.
With these configuration and settings, externally applying a direct current control voltage to the input terminal varies the oscillation frequency depending on the magnitude of the control voltage. For example, the magnitude of the control voltage is inversely proportional to the capacitance value of the variable capacitance diode. Therefore, a high control voltage produces a high oscillation frequency. Since the bias circuit regards the input terminal and the oscillator circuit as being separate from each other, the influence of the rectification of the variable capacitance diode in the oscillator circuit can be successfully reduced. Thus, the range of logically obtained oscillation frequency can approximately match the range of actual oscillation frequency.
2. Description of the Prior Art
With the increasing demand for multimedia and digital audio units, signal synchronization at a high frequency is strongly required.
To meet this demand, the voltage control oscillator is used with the PLL circuit. FIG. 1 shows the configuration of the conventional voltage control oscillator. The voltage control oscillator shown in FIG. 1 comprises a resonator 1 including an inductive resonator such as a coil; an active oscillator circuit 2 such as a transistor and an IC; a variable capacitance diode 3; an impedance element 4 (hereinafter referred to also as a resistor) for limiting the current through the variable capacitance diode; and input terminals 5a and 5b (input terminal 5b is grounded).
With the above listed configuration, applying a direct current voltage to input terminals 5a and 5b allows the control voltage to be applied to the variable capacitance diode 3 through the impedance element 4. The capacitance value of the variable capacitance diode 3 alters with the magnitude of the control voltage applied as a reverse bias voltage. That is, the magnitude of the control voltage is inversely proportional to the capacitance value. Therefore, a high control voltage produces a high oscillation frequency.
Thus the oscillation frequency varies with a change in an external control voltage within a predetermined range.
The required performance of the voltage control oscillator is good stability and a wide variable frequency range, that is, a wide frequency variation within the range of a predetermined control voltage.
However, if the control voltage is altered from 0 V to 5 V for the conventional voltage control oscillator, the oscillation frequency does not vary as low as a preliminarily calculated value at a lower voltage. FIG. 2 shows the variations of the oscillation frequency in relation to the control voltage. In FIG. 2, the vertical axis indicates normalized frequency variations (.DELTA.f/f where f indicates an oscillation frequency; .DELTA.f indicates the change amount in the oscillation frequency and is represented by ppm), while the horizontal axis indicates a control voltage. The solid curve represents the actual variations of the frequency, and the dashed curve represents the ideal variations.
FIG. 2 shows the frequency variations when a given control voltage ranges from 0 V to 5 V. Ideally, the frequency should vary as shown by the dashed curve in FIG. 2. That is, the frequency variations should range from approximately -4200 ppm to +1500 ppm corresponding to the control voltage from 0 V to 5 V respectively. Thus, the frequency ideally varies as shown by the dashed curve in FIG. 2 if the control voltage alters from 0 V to 5 V.
However, in this example, the frequency actually varies from approximately -3200 ppm to +1500 ppm corresponding to the range of the control voltage from 1.48 V to 5 V. That is, the frequency is not so low as the value calculated for the control voltage of 0 V but is lowered to the frequency of approximately -3200 ppm at the control voltage of 1.48 V even if the control voltage of 0 V is externally applied in practice.
This is because an offset voltage of approximately 1.48 V has been generated at the cathode of the variable capacitance diode 3 even when the control voltage of 0 V is externally applied. The offset voltage of approximately 1.48 V is generated at the cathode of the variable capacitance diode 3 on the following grounds even when the control voltage of 0 V is externally applied.
FIG. 3 shows the circuit equivalent to the oscillator unit shown in FIG. 1. The circuit comprises a diode D, a capacitor C, a coil L, and a negative resistor R. If the equivalent circuit oscillates at 10 MHz, then oscillation current I flows through the circuit and the diode D apparently performs detection and rectification processes and generates as a result a direct current at the cathode of the diode D (approximately 1.48 V in this example). That is, as shown in FIG. 4, the rectification at the oscillation at 10 MHz produces the potential of approximately 1.48 V relative to the ground level G at the cathode of the diode D.
FIG. 5 shows this operation as actual data obtained through an oscilloscope. In FIG. 5, the vertical axis indicates the potential at the cathode of the diode D, while the horizontal axis indicates time. When the control voltage is 0 V, the potential at the cathode of the diode D should be the ground level G, but is 1.48 V actually.
Thus, the voltage of approximately 1.48 is applied as a reverse bias voltage at the cathode of the variable capacitance diode 3 even if the control voltage of 0 V is externally applied in practice. Therefore, even if the control voltage of 0 V is assumed, the reverse bias voltage of 1.48 V is actually applied to the variable capacitance diode 3. Therefore, the variations of the oscillation frequency range correspondingly and show the range narrower than the expected range.
The important function of a voltage control oscillator is to cover a wide range of oscillation frequency within a predetermined voltage range. With the above described conventional voltage control oscillator, the oscillation frequency cannot be lowered to the expected level at a lower control voltage and the actual range of the oscillation frequency is narrower than the calculated range.